Method and apparatus for sealing a chip carrier and lid

ABSTRACT

A chip carrier and lid are sealed by mounting the chip carrier in an inverted position and mounting a lid having a sealing preform in an inverted position beneath and facing the chip carrier. The chip carrier and lid are then heated to melt the sealing preform and the chip carrier and lid are moved together to join them at the sealing preform.

FIELD OF INVENTION

[0001] This invention relates to a method of sealing a chip carrier andlid and an apparatus for holding, heating and joining them.

BACKGROUND OF INVENTION

[0002] Micro-mechanical gyroscopes and accelerometers must be sealed ina vacuum or inert atmosphere to operate. Two methods of vacuum sealingsensors are in use. In one method a lid with a small hole in it iswelded on a chip carrier after a sensor is mounted inside. The chipcarrier is then placed in a vacuum chamber and heated for several hoursto drive out moisture and activate a getter element. Sealing isaccomplished by dropping an indium ball on the lid, which melts andplugs the small hole as the chip carrier cools under vacuum. In thesecond method, oversized lids and chip carriers are placed incompartments of graphite trays. The lid tray is suspended above the chipcarrier tray in a vacuum chamber and heated by passing an electriccurrent through it. At the end of the degas time, the chip carrier trayis raised. Holes in the bottom of the lid tray allow the chip carriersto contact the lids. When this occurs, a braze preform on the chipcarrier melts and seals the lid to the chip carrier. These methods ofsealing require significant touch labor and processing time. They alsoyield packages that are less than sufficiently robust for militaryapplications. The indium ball seal can soften and leak at temperaturesabove 125° C., while the oversize lid flexes and cracks its braze sealwhen subjected to high acceleration, e.g., a gun launch. The chipcarrier with the weldable seal ring that is used in the indium ballprocess is expensive. In addition, in both approaches the sealingpreform is added to the chip carrier after the chip is bonded into thecarrier so that the chip can be damaged or contaminated during thatprocess.

BRIEF SUMMARY OF THE INVENTION

[0003] It is therefore an object of this invention to provide animproved method of sealing a chip carrier and lid and an apparatus forholding, heating and joining them.

[0004] It is a further object of this invention to provide an improvedmethod of sealing a chip carrier and lid and an apparatus for holding,heating and joining them which requires less touch labor and processingtime and is less expensive.

[0005] It is a further object of this invention to provide an improvedmethod of sealing a chip carrier and lid and an apparatus for holding,heating and joining them which yields a more robust product.

[0006] It is a further object of this invention to provide an improvedmethod of sealing a chip carrier and lid and an apparatus for holding,heating and joining them which is better sealed, less likely to leak andcan withstand higher acceleration force.

[0007] It is a further object of this invention to provide an improvedmethod of sealing a chip carrier and lid and an apparatus for holding,heating and joining them which avoids the risk of contamination anddamage attendant on applying the sealing preform to the carrier afterthe chip is bonded in place.

[0008] The invention results from the realization that a truly simple,yet effective sealing of a chip carrier and lid can be achieved bymounting the chip carrier in an inverted position with the lid mountedinverted, below and facing the chip carrier, then heating them andbringing them together to join them at the sealing preform carried bythe lid.

[0009] This invention features a method of sealing a chip carrier andlid including mounting a chip carrier in an inverted position andmounting a lid having a sealing preform in an inverted position beneathand facing the chip carrier. The chip carrier and lid are heated to meltthe sealing preform and the chip carrier and lid are moved together tojoin them at the sealing preform.

[0010] In a preferred embodiment there may be an evacuating space aroundthe lid and carrier, the chip carrier may be heated to degas it, and thelid may include a getter. The method further includes heating the getterto activate it to evacuate the chip carrier. The moving of the chipcarrier and lid together may include lifting the lid out of its mountinginto contact with the carrier.

[0011] This invention also features a chip carrier and lid sealingapparatus including an upper holder for mounting at least one chipcarrier in an inverted position and an upper heater for heating the chipcarrier. A lower holder mounts at least one chip carrier lid with asealing preform in an inverted position facing the chip carrier and alower heater heats the lid and sealing preform. A drive mechanism movesthe upper and lower holders together to join the carrier and lid withthe sealing preform.

[0012] In a preferred embodiment the upper holder may include a cavityfor receiving the chip carrier and a retainer for securing the chipcarrier in the cavity. The retainer may include a keeper block and aspring for urging the keeper block against the chip carrier to secure itin its cavity. The lower holder may include a cavity for receiving thechip carrier lid. There may be a degas heat source for heating the chipcarrier to degas it. The drive mechanism may include a drive device formoving at least one of the holders toward the other. The drive devicemay move the upper holder toward the lower holder. The drive mechanismmay include a lifter device for lifting the lid out of the cavity tomeet the chip carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects, features and advantages will occur to thoseskilled in the art from the following description of a preferredembodiment and the accompanying drawings, in which:

[0014]FIG. 1 is a flow chart of a method of sealing a chip carrier andlid according to this invention;

[0015]FIG. 2 is a diagrammatic front elevational view with portions insection of a chip carrier and lid apparatus in the open positionaccording to this invention;

[0016]FIG. 3 is a top plan view of the apparatus of FIG. 2;

[0017]FIG. 4 is a view similar to FIG. 2 with the apparatus in theclosed position;

[0018]FIG. 5 is a diagrammatic plan view of a portion of the chipcarrying tray in FIGS. 2 and 4 with a retainer according to thisinvention; and

[0019]FIG. 6 is a diagrammatic three-dimensional view of the retainerand chip carriers in the recesses of the tray as shown in FIG. 5.

[0020] There is shown in FIG. 1 one embodiment of the method accordingto this invention which obtains the benefits and advantages of thisinvention independent of the particular apparatus used to carry it out.A chip carrier is mounted in an inverted position 10 and then a lid witha getter and sealing preform is mounted in an inverted position beneathand facing the chip carrier 12. The space around the lid and chipcarrier are evacuated 14. The chip carrier is then heated to degas it16. The lid is then heated to activate the getter 18 and the chipcarrier and lid are then heated to melt the sealing preform 20 and thecarrier and lid are moved together to be joined at the sealing preform22. The getter material may be any suitable material such as SAES ST 122which can be activated at a temperature of 400° C. for fifteen minutesor 350° C. for 360 minutes, or the getter material may be SAES 707activated at a temperature of 500° C. for fifteen minutes or 400° C. for360 minutes. The brazed material may be, for example, a brazing materialsuch as gold-tin which functions at 260° C., tin-silver at 240° C., orgold germanium at 320° C.

[0021] An apparatus according to this invention for carrying out themethod includes a chip carrier and lid sealing system 50, FIGS. 2 and 3.There is a ball slide 52 including housing 54 which contains a leadscrew 56 driven by motor 58 to move carriage block 60 up and down inhousing 54. Mounted for movement with carriage block 60 is upper supportframe 62 which contains upper heater 64 that may be a stainless steeltable with one or more cartridge heaters. A tray 66, also typically madeof stainless steel, is mounted in inverted position against heater 64.Tray 66 has one or more cavities 68 a, 68 b which receive chip carriers70 a, 70 b which contain the chips 72 a, 72 b that are wire-bonded toand form a part of the chip carriers. A retainer 74, shown in moredetail in FIGS. 5 and 6, includes retainer 74 including one or moreretainer blocks 76 a, 76 b which are urged outwardly by spring 78 tourge carriers 70 a and 70 b against the walls of their cavities 68 a and68 b and secure them in position. A retaining pin 80 holds pin 78 inposition. Lower support frame 82 includes stainless steel tray 84 whichincludes one or more cavities 86 a, 86 b for holding lids each of whichincludes a getter 90 a, 90 b and a sealing preform 92 a, 92 b. A lowerheater table 94 provides the heat to heat getters 90 a, 90 b and sealingpreforms 92 a, 92 b. In this embodiment lower support frame 82 is fixedand as previously indicated upper support frame 62 moves. Alternatively,the converse could be true or both could move. System 50 may be includedin bell jar 129 in order to provide a vacuum in the space around lids 88and chip carriers 70.

[0022] A lifter mechanism 96 includes a jack plate 98 which carries oneor more lifter pins 100 a, 100 b which when jack plate 98 rises, movesthrough holes 102 a, 102 b to lift lids 88 a, 88 b out of theirrespective cavities 86 a, 86 b so that at least the sealing preforms 92a, 92 b clear the top of tray 84. Thus upon heating sealing preforms 92a and b will not inadvertently join to the tray 84 while they areattempted to be brazed to their respective chip carriers 70 a and b.Jack plate 98 is driven up and down by means of a conventional scissormechanism 104 with a center pivot 106 and one end fixed at 108 while theother end 110 is mounted to carriage block 112 which is driven to moveleft and right on lead screw 1114 driven by motor 116. As can beunderstood when carriage block 112 moves to the left scissor 104 risesand lifts jack 98 and jack pins 101 a and b, and the converse is truewhen carriage block 112 moves to the right. An additional heater in theform of an infrared source 120 may be employed using fiber opticelements 122 a and 122 b to apply further heat to chip carriers 70 a andb in order to degas them. The temperature of the heaters and the timefor which the heat is applied is as explained previously with respect toFIG. 1. A time and temperature control 126 is used to control thetemperatures and the periods for which they are applied. In addition, aslide mechanism 128 driven by a motor not shown is used to move the heatsource 120 and fiber optic elements 122 a and 122 b out of the way afterdegassing has been completed so that the upper and lower support frames62, 82 and their trays 66 and 84 may be moved toward each other withoutinterference.

[0023] The final stage where the upper tray 66 and lower tray 84 arebrought together without touching is shown in FIG. 4, where it can beseen that at that point the jack pins 100 a and b have raised lids 88 aand b sufficiently so that their sealing preforms 86 a and b clear thetop of tray 84. Springs 101 a, 101 b provide for constant complianceagainst lids 88 which are quite thin, typically in the range of twentythousandths of an inch thick, in order to minimize their mass and theeffect on them of elevated accelerations. Preliminary data indicatesthat the final product is able to withstand accelerations of 40,000 toeven 100,000 gs. Holes 127 a and b are provided to allow chip carriers70 a and b to be popped out should they become lodged in theirrespective cavities 68 a and b.

[0024] One construction of the retainer mechanism 74 which may be usedto hold four chip carriers 88 a-d in place upside down in tray 66 isshown in FIGS. 5 and 6, where it can be seen that retainer mechanism 74includes a pair of hemispherical blocks 76 a and b that are urged apartby spring 78 which is secured from falling out by means of pin 80. Theforce of spring 78 on blocks 76 a and b causes them to provide a forceagainst the respective comers of chip carriers 88 a-d which drives theminto the reference edges of cavities 68 a-d. One or more springs 130,132 may be used to keep blocks 76 a and b from coming apart when thereare no chip carriers 88 a-d in place.

[0025] Although specific features of the invention are shown in somedrawings and not in others, this is for convenience only as each featuremay be combined with any or all of the other features in accordance withthe invention.

[0026] Other embodiments will occur to those skilled in the art and arewithin the following claims:

What is claimed is:
 1. A method of sealing a chip carrier and lidcomprising: mounting a chip carrier in an inverted position; mounting alid having a sealing preform in an inverted position beneath and facingthe chip carrier; heating the chip carrier and lid to melt the sealingpreform; and moving the chip carrier and lid together to join them atthe sealing preform.
 2. The method of claim 1 further includingevacuating the space around the lid and carrier.
 3. The method of claim1 further including heating the chip carrier to degas it.
 4. The methodof claim 1 in which the lid includes a getter and the method furtherincludes heating the getter to activate it to evacuate the chip carrier.5. The method of sealing a chip carrier lid of claim 1 in which movingthe chip carrier and lid together includes lifting the lid out of itsmounting into contact with the carrier.
 6. A chip carrier and lidsealing apparatus comprising: an upper holder for mounting at least onechip carrier in an inverted position; an upper heater for heating thechip carrier; a lower holder for mounting at least one chip carrier lidwith a sealing preform in an inverted position facing the chip carrier;a lower heater for heating said lid and sealing preform; and a drivemechanism for moving said upper and lower holders together to join thecarrier and lid with said sealing preform.
 7. The chip carrier and lidsealing apparatus of claim 6 in which said upper holder includes acavity for receiving the chip carrier and a retainer for securing thechip carrier in said cavity.
 8. The chip carrier and lid sealingapparatus of claim 7 in which said retainer includes a keeper block-anda spring for urging said keeper block against a chip carrier to secureit in its cavity.
 9. The chip carrier and lid sealing apparatus of claim6 in which said lower holder includes a cavity for receiving the chipcarrier lid.
 10. The chip carrier and lid sealing apparatus of claim 6further including a degas heat source for heating the chip carrier todegas it.
 11. The chip carrier and lid sealing apparatus of claim 6 inwhich said drive mechanism includes a drive device for moving at leastone of said holders toward the other.
 12. The chip carrier and lidsealing apparatus of claim 11 in which said drive device moves saidupper holder toward said lower holder.
 13. The chip carrier and lidsealing apparatus of claim 9 in which said drive mechanism includes alifter device for lifting the lid out of said cavity to meet the chipcarrier.